Relayed COSY Experiments

Relayed COSY experiments are used to establish a link between separate homonuclear spin systems either via a heteronuclear relay step (left side of Fig. 5.24) or in H, H spin systems a homonuclear relay step (right side of Fig. 5.24). In the latter case correlation peaks are generated for protons which are not directly coupled together but form part of the same spin systems. Correlation s can arise using the small value long-range coupling constants J(H, H) and J(H, H). 

Even though IH coherences are assigned to both dimensions, heteronuclear relay COSY experiments can be helpful in the indirect detection of heteronuclear functional group. Often from the relay cross peak the functional group acting as a bridge between two spin systems can be deduced. 

Purpose / Homonuclear correlation of signals which are not directly related 

Crotonaldehyde has been used in the literature to test the homonuclear and heteronuclear relayed COSY experiment [5.142]. In the following Check its a spin system based on crotonaldehyde will also be used but because the maximum number of coupled spins in a cluster is restricted to nine, see section 1.2, and to speed up the calculation the CH3 group has been replaced by a CHX2. Two spin systems have also been created for this crotonaldehyde type molecule one for a pure isotopomer and a second with the natural ratio of and DC isotopomers the latter being used for pulse sequence with relay transfer through the DC nucleus. 

The test spin system corresponds to an IH AKMX system. H-1 has a single coupling to H-2 while H-2, H-3 and H-4 couple with each other with an observable scalar coupling. So in the homonuclear relay COSY experiment proton H-1 should exhibit connectivity to either H-3 or H-4 or both. 

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The relayed-COSY experiment shall be considered only very briefly because it has essentially been superseded by the far superior TOCSY experiment described below, although one may still encounter references to this experiment in older literature however. The relayed-COSY experiment (Fig. 5.57) attempts to overcome problems caused by coincidental overlap of crosspeaks in COSY that can lead to a breakdown in the stepwise tracing of coupling networks within a molecule. It incorporates an additional coherence transfer step in which that transferred from a spin. A, to its partner, M, as in the standard COSY, is subsequently relayed onto the next coupled spin X in the sequence. This produces a crosspeak in the spectrum between spins A and X even though there exists no direct coupling between them, by virtue of them sharing... 

Fig. 5.25 Molecular structure of the relay COSY experiment test spin system.

The H-X-H relay COSY experiment enables the verification that two apparently isolated IH spin systems belongs to the same molecule. The experiment is based upon the detection of a heteronuclear coupling interaction between at least two protons from each spin system to the same bridging heteronucleus such as a l C atom in a carbonyl functional group. By default the experiment is not very sensitive, the relay cross peak depending on the magnetization transfer to a 1 C atom with low natural abundance. 

Load the configuration file ch54113.cfg an6 run the simulation of the 2D 130-1H relay COSY experiment of the crotonaldehyde type spin system. In this modified spin system the coupling between H-1 and H-2 is excluded to show the relay transfer from H-1 to H-2 trough C-1. Compare the result with the basic 1H COSY spectrum. Repeat both simulations using the spin system relcspsy.ham. Note however that in this spin system the 13C nucleus is 100% abundant. 

As shown in the list of relay COSY experiments heteronuclear correlation experiments are possible. In Check it 5.4.1.14 the H-H-X relay IR, DC COSY experiment for the crotonaldehyde type spin system is calculated. Implementing a relay step to a heteronucleus enables complex IR COSY spectrum to be disentangled by including a heteronuclear polarization transfer to link the IR signals to the heteronuclear chemical shift dimension. Fig. 5.26 illustrates this schematically for two spin systems. Since 8(Ra) = 5(Rd) and 5(Rt>) = 5(Rg) at least two IR, IR relay cross peaks which belong to two different relayed spin systems overlap in the IR, IR spectrum. Rowever because 5(Cc) 4 5(Cf) these peaks may be separated if the correlation peaks can be related to the heteronucleus which has a different chemical shift for each peak. 

An alternative to the H-H-X relay COSY experiment is the X-H-H relay COSY experiment [5.148], which has the advantage that it is based on the detection of the sensitive nucleus [5.3]. In Check it 5.4.1.15 the X-H-H relay COSY sequence is created and then various aspects of the phase cycling examined to illustrate how the original phase cycling proposed in the literature must be adapted to give quadrature detection in fl [5.148] using NMR-SIM. 

The relay COSY experiments has to contend with reduced signal intensity during the long relay step. In contrast in a TOCSY experiment because of the different effective rate constant during the spinlock period relaxation effects are minimized [5.143] and TOCSY experiments can be successfully performed on fast relaxing molecules such as polymers or organic macromolecules. 

To circumvent the difficulty, one uses the relayed COSY experiment in which the coupling from H-2 to H-3 rebovmds on H-1 through the H-1 to H-2 J relay. In this case there is a "relayed correlation" between H-1 and H-3 in the absence of genuine couphng between these protons. 

The disaccharide 3,2 -0-isopropylidene acetals (31) each contain a trioxocane ring. Using one-and two-dimensional H NMR techniques, including partially relaxed COSY and RELAY-COSY experiments, the ring protons were assigned and their values and coupling constants are shown in Table 5 <89JCS(P2)1867>. 

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